These sensors are used for example in scanners. They comprise a linear array of several parallel rows of photosensitive pixels. The sequencing of the control circuits of the various rows (control of the exposure time and then control of the readout of the photogenerated charges) is synchronized with respect to the relative scrolling between the scene and the sensor, so that all the rows of the sensor see in succession the same row of the observed scene. The signals generated by each row are then added point by point for each point of the row observed.
For a constant exposure time, the sensitivity of the sensor is improved in proportion to the number N of rows, or else, for a constant sensitivity, the exposure time may be divided by N. This number N may be, for example, 16 or 32 for industrial-control applications or for space-based Earth observation or even from 60 to 100 rows for medical applications (dentistry, mammography, etc.).
The signal-to-noise ratio is improved in proportion to the square root of the number N of rows of the sensor.
In addition, the sensitivity non-uniformity of the pixels in the same array, and the dark current non-uniformity of the pixels are reduced due to the averaging that results from adding the signals from the various rows.
In CCD (charge-coupled device) sensors, the pointwise addition of the signals is achieved simply by transferring to a row of pixels the charges generated and accumulated in the preceding row of pixels, synchronously with the relative movement between the scene and the sensor. The last row of pixels, having accumulated the charges generated by the observed image row N times, may be read out.
The usual CCD image sensor technology is relatively costly—it uses high supply voltages and consumes a substantial amount of energy. This technology is based on the use of adjacent, mutually-overlapping polysilicon gates.
Image sensor technology has since moved towards transistor-based active-pixel sensors, called from now on CMOS sensors for simplicity because they are generally fabricated in CMOS (complementary metal-oxide-semiconductor) technology. In these CMOS sensors there is no longer charge transfer from row to row towards a readout circuit or a register but instead transistor-based active pixels that gather photogenerated electrical charges and convert them directly into a voltage or a current. The various rows of the sensor therefore successively supply voltages or currents representative of the illumination received by the row. These structures do not allow noiseless addition of these currents or voltages and it is therefore difficult to produce a time-delay and integration charge sensor. The fabrication technology is however simple, consumes little power, and uses low voltages.
Attempts have however been made to fabricate TDI CMOS sensors.
In particular, switched capacitors in which successive received currents are integrated, thus accumulating in one and the same capacitor charges received from several pixels in a column, have been tried (U.S. Pat. No. 6,906,749, WO 01/26382).
It has also been proposed to convert the signals from a row of pixels into digital values and sum the digital value corresponding to the pixel of rank j of the row in an accumulating register of rank j that accumulates the numerical values corresponding to the pixels of one and the same rank j of N successive rows (patent FR 2 906 080).
In patent FR 2 906 081 it was proposed to apply, to the photodiode of a pixel in one row, the output voltage of a pixel in a preceding row so as to copy thereto the charges of the preceding pixel before isolating the photodiode and integrating new charges resulting from the light. Hence, at the end of an integration time the photodiode comprises the sum of the charges corresponding to the preceding row and the new integrated charges. This operation however induces transfer noise that reduces the signal-to-noise ratio.
Finally, solutions using charge accumulation in the pixel have been proposed, for example in the patent application US 2008/0217661. They use a more complicated technology than is strictly necessary for fabricating image sensors in CMOS technology or they induce losses during charge transfer.
Attempts to fabricate a linear TDI sensor using a technology simpler than the customary CCD technology have therefore not been satisfactory.
The object of the invention is to provide a more advantageous solution for fabricating sensors, operating according to the principle of charge-transfer structures but using a technology compatible with CMOS-technology circuits, and notably a technology that uses only a single polycrystalline silicon gate level and not a double, overlapping gate level as is the case in conventional CCD technologies.